Global warming is a serious problem for all humankind, and in order to delay and stop the progress of the warming, every country is trying to promote energy saving by building efficient power systems or smart grids, and promote large-scale introduction of natural energies that do not discharge carbon dioxide, such as photovoltaic power generation and wind power generation, etc.
For example, the wind power generation has the merit of not discharging carbon dioxide by utilizing natural energy that circulates in the atmosphere; however, there is the concern that the power system may be adversely affected or the quality of power may be deteriorated because the output of the power generation is dependent on wind conditions and accordingly unstable. In order to prevent such an adverse influence on the power system and utilize the energy effectively, it is expected that a wind power generation storage system is achieved in which energy fluctuations are equalized such that the output of the power generation is stabilized by using a lead acid storage batttery, etc. The lead acid storage batttery used for suppressing the fluctuation in the output of the wind power generation is required to have the same long life as a wind power generator and be installed at a low cost.
The lead acid storage batttery for suppressing the fluctuation in the output of the wind power generation is operated in a PSOC (Partial State of Charge) such that charge/discharge can be performed in accordance with the fluctuation in the output of the wind power generation. Accordingly, the lead acid storage batttery is not fully charged in a normal operation state, different from the conventional lead acid storage batttery for emergency that is fully charged normally and discharged when needed and industrial lead acid storage batttery that is fully charged at night and discharged when a load is large in the daytime. In order to prevent the degradation of the negative electrode due to sulfation occurring in a low SOC in the particular application, equalized charge (recovery charge) is regularly (usually once every one or two weeks) performed in which the lead acid storage batttery is fully charged. Another purpose of the equalized charge is, for accurately grasping an SOC, to regularly reset an SOC value such that the value is made to be 100% after the equalized charge.
However, there has been a problem that, if the equalized charge is performed too often, degradation of the positive electrode conversely occurs due to the overcharge, and thereby the life of the lead acid storage batttery is shortened, etc. Patent document 1 discloses the case where equalized charge intervals of a lead acid storage batttery are changed in accordance with ambient temperature. Patent document 2 discloses that the degradation of the positive electrode is prevented by setting the amount of the overcharge at the equalized charge of a lead acid storage batttery to a lower value (99% to 102%) than the conventional one (110% to 115%).